gh-151815: Fix segfault in template_iter on allocation failure

[timurmamedov1]

template_iter() allocates a templateiterobject with PyObject_GC_New (which does not zero memory), but only assigns stringsiter and interpolationsiter after both PyObject_GetIter calls succeed. If either call fails, Py_DECREF(iter) runs templateiter_clear, which calls Py_CLEAR on uninitialized pointers, causing a segfault.

Fix: initialize both fields to NULL immediately after allocation so Py_CLEAR is safe on the error paths.

• Issue: Segfault: dealloc of uninitialized iterator in template_iter (Objects/templateobject.c:232) #151815

[Copilot]

Pull request overview

Fixes a crash in the C implementation of t-string Template iteration by making the iterator object safe to deallocate when construction fails partway through (e.g., on PyObject_GetIter() allocation failure).

Changes:

• Initialize templateiterobject’s stringsiter and interpolationsiter to NULL immediately after PyObject_GC_New() so tp_clear/Py_CLEAR is safe on error paths.
• Add a NEWS blurb documenting the crash fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Objects/templateobject.c	Prevents segfault by NULL-initializing iterator fields before any fallible operations.
Misc/NEWS.d/next/Core_and_Builtins/2026-06-20-14-00-00.gh-issue-151815.TmplIt.rst	Documents the crash fix in the Core and Builtins NEWS entries.

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[sergey-miryanov]

I would suggest moving allocation after getting interpolationsiter.

[timurmamedov1]

Done, moved the allocation after both GetIter calls.